A good general rule of thumb is 4 horsepower for every 1 foot of mower. It really depends though on how tall you let the grass get and how fast you want to go. If your tractor is 24hp I would probably recommend a 6' or 7'.
I have an F215 which is almost exactly the same size tractor and find that a 5ft finish mower is an ideal compromise. The mower can be picked up without adding weight to the front of the tractor and it cuts any length of grass without bogging down and the whole set up is very manouverable and balanced. I am also sure that it would drive a 6ft without problem.
I vote for a 5'. Mine is a 2002d (20pto hp) and it is a good match. No weight problems or power problems. It is possible to load it up if you mower pastures with it. Finish mowers are made for just that, "finish".
